It is a general goal within the vehicle industry to reduce fuel consumption of vehicles, both to save natural resources and to reduce exhaust emissions. The fuel consumption of a motor vehicle can be reduced not only by optimizing the efficiency of the vehicle's engine, but also by e.g. reducing the weight of the vehicle and its aerodynamic drag. Even once the design of an engine and the vehicle to which it is been fitted has been established, it is still possible to affect the actual fuel consumption of the vehicle by ensuring that the engine is placed under load only when absolutely necessary.
In this respect, it is to be observed that the engine of a vehicle is employed not only to propel the vehicle, but also to drive ancillary equipment such as pumps and electrical generators. A typical example of a piece of ancillary equipment for a commercial vehicle is an air compressor for charging an air tank used to power the vehicle's braking system. Upon starting the engine of a vehicle equipped with such a braking system, the air compressor is driven by the engine until at least a minimum operating value of air pressure is attained in the air tank. Thereafter, the air pressure in the tank is maintained between predetermined maximum and minimum values by recharging the tank each time the air pressure drops below the minimum value. Naturally, each time the air compressor is switched in, a load is placed on the engine.
It is known from e.g. EP-A-0 335 086 to provide a system which detects when a vehicle engine is not being used to propel the vehicle and to allow ancillary equipment to be switched on under such a condition. Thus, in said document, a sensor detects when the throttle pedal is released, thereby indicating that no additional propulsive force is presently required by the driver, and thereafter allows ancillary equipment to be switched on should activation of the equipment be necessary. For example, if the air pressure in the air tank is at a low level, once the throttle is released, the air compressor is switched on. Since the engine is not being used to propel the vehicle, it will be the kinetic energy of the vehicle which drives the air compressor. As such, the engine requires no additional fuel to compensate for the load placed on the engine by the air compressor.
Although the system described above makes use of the kinetic energy of the vehicle to power items of ancillary equipment, the switching on of the ancillary equipment will increase the engine braking effect of the engine on the vehicle. As a consequence, a condition may arise in which the driver of the vehicle, when anticipating the need to reduce the speed of the vehicle somewhat due to the fact that e.g. the vehicle is approaching a corner, lifts his foot from the throttle to allow the vehicle to coast under normal engine braking. If, however, the lifting of his foot from the throttle pedal should result in a piece of ancillary equipment switching on, then the degree of engine braking will be greater than that anticipated by the driver. As a consequence, the driver may well need to reapply the throttle to ensure that the vehicle reaches the corner at the originally intended speed.
Since every application of the throttle implies an increase in the consumption of fuel, it would be desirable to provide a system which could ensure that increased engine braking due to the switching on of ancillary equipment occurs only when it is desirable to significantly reduce the kinetic energy of the vehicle.